This invention relates to the preparation of flame retardant polycarbonates and more particularly to phosphine oxide containing polycarbonates.
Polycarbonates are well known as tough, clear, highly impact resistant thermoplastic resins. It is desirable to have polycarbonates possess flame retardancy. Several flame retardant agents are known to be effective. It is known to use alkali metal salts of strong sulfonic acids as flame retardant agents, however, the increased hydrolytic sensitivity of the polymer matrix presents some difficulties. When alkali metal salts of strong sulfonic acids are used, it is also necessary to use drip inhibitors and often gas phase flame retardant agents. Drip inhibitors such as Teflon.RTM. are used, but the resulting polymers lose clarity. Gas phase inhibitors such as halogenated flame retardants have also been used. The use of halogens presents problems with corrosion and toxicity. As an alternative to halogenated compounds, phosphorus containing compounds such as triphenylphosphate have been used. Most effective phosphorus compounds are soluble in polycarbonates, and the resulting polymer blends have low glass transition temperatures (T.sub.g) and low impact resistance compared to the base resin.
The present invention is based on the discovery that certain dihydroxyarylphosphine oxide units can be incorporated into polycarbonates to form polymers, and these polymers unexpectedly retain high T.sub.g values similar to polycarbonates not containing these dihydroxyarylphosphine oxide units. In addition to retaining high T.sub.g values, the phosphine oxide substituted polycarbonates exhibit improved flame retardancy as shown by high limiting oxygen index values.